The present invention relates to a drive-slip control device (ASR) and an anti-lock system (ABS) for a road vehicle with a hydraulic multi-circuit brake system, in which the risk of failure of the brake circuit of the driven wheels is considerably reduced.
DE 3,802,133 A1 shows a known ASR in combination with an ABS. There, the ASR works on the principle of decelerating again a driven vehicle wheel tending to spin by activating its wheel brake, and the ABS works on the return-flow principle at least on the brake circuit assigned to the driven vehicle wheels. The return-flow pump of the ABS assigned to the brake circuit of the driven vehicle wheels is used for the ASR as an auxiliary-pressure source, the outlet pressure of which is fed, in brake-pressure build-up phases of the drive-slip control, into the wheel brake or wheel brakes subjected to the control.
Brake-pressure regulating valves of the ABS are used for a similar purpose within the ASR. As long as the ASR is activated, the outlet-pressure space assigned to the brake circuit of the driven vehicle wheels and belonging to the brake unit conventionally designed as a tandem master cylinder is shut off by an ASR control valve from the main brake line of the brake circuit of the driven vehicle wheels. The basic position of this ASR control valve is its throughflow position assigned to the normal braking mode and to the braking mode assigned to an anti-lock control. A pressure relief valve is connected in parallel with this ASR control valve, via which, in the drive-slip control mode, overpressure generated in the main brake line of the brake circuit of the driven vehicle wheels can be relieved towards the brake unit and via this to the pressureless reservoir of the brake system.
A buffer accumulator is connected to the return line of the brake circuit of the driven vehicle wheels and is designed as a low-pressure accumulator. In brake-pressure reduction phases of the anti-lock control and of the drive-slip control, the buffer accumulator quickly receives brake fluid bled from one or both wheel brakes of the brake circuit of the driven vehicle wheels, before this fluid is successively pumped back by the return-flow pump of this brake circuit into the outlet-pressure space assigned thereto of the brake unit or the brake-fluid reservoir of the brake system in a plurality of feed strokes of the return-flow pump.
The buffer accumulator can be charged to a pressure of approximately 15 bars at the end of a pressure reduction phase of the drive-slip control. If braking occurs at such a moment when the buffer accumulator is being charged, with the result that the drive-slip control mode is interrupted, and if this braking takes place in a driving situation in which, even with only a low brake pressure, an anti-lock control on the driving axle of the vehicle also becomes necessary, for example because the vehicle is traveling on a road made smooth from ice or snow then, since the buffer accumulator is full, during the introductory brake-pressure reduction phase of the anti-lock control, brake pressure either cannot be reduced at all or at best be reduced much too slowly.
To prevent a danger that the rear wheels of the vehicle will lock as a result of this inability to reduce the brake pressure, the known control device has a 2/2-way solenoid valve inserted between the buffer accumulator and the brake-fluid reservoir of the brake system. In brake-pressure reduction phases of the drive-slip control, the solenoid valve is moved into its throughflow position so that brake fluid can be bled from the buffer accumulator directly towards the brake-fluid reservoir. In the event of a malfunction of this accumulator bleeding valve, namely leakage or catching in the throughflow position, therefore, there is a possibility that the brake circuit of the driven vehicle wheels may be emptied when the anti-lock control responds to it and therefore fails. This possibility constitutes a considerable safety risk.
An object of the present invention is, therefore, to improve an ASR combined with an ABS, such that, even with a nevertheless low total production outlay therefor, the risk of failure of the brake circuit of the driven vehicle wheels is considerably reduced.
According to the present invention, this object has been achieved by providing a system in which, after termination of a situation requiring the drive-slip control and in the event of an interruption of a drive-slip control cycle as a result of the actuation of the brake system, an electronic control unit generates control signals by way of which for a delay time the return-flow pump continues to operate and the drive-slip control valve is maintained in a closed position assigned to the drive-slip control mode, the duration of this delay time being sufficient to ensure that a quantity of brake fluid corresponding to a quantity previously received by the buffer accumulator of the brake circuit of the driven vehicle wheels is conveyed out of the brake circuit back into the outlet-pressure space of the brake unit assigned thereto.
Because the return-flow pump is cut out with a delay in relation to the termination of a situation requiring the drive-slip control and the ASR control valve is reset to its basic position assigned to the braking mode, the buffer accumulator can be emptied sufficiently quickly by the return-flow pump driven for a delay time even with the brake circuit of the driven vehicle wheels permanently closed. The risk of a failure of the brake circuit by the actuation of the anti-lock control on the brake circuit of the driven vehicle wheels is thus avoided.
Furthermore, a simpler overall design is generally achieved since an accumulator bleeding valve is not required. If a drive-slip control cycle is interrupted as a result of the actuation of the brake system, then as long as the ASR control valve is still closed the return-flow pump works from the buffer accumulator into the wheel brakes, so to that extent, although these are shut off from the main brake cylinder, brake pressure can be built up.
When the delay time is determined by a timer of the electronic control unit, in the simplest instance this time will have a fixed value, e.g. 400 ms is suitable.
Other features of the present invention give advantageously simple alternatives for obtaining a variable duration of the delay time appropriate to particular requirements.
By way of a solenoid valve which can be produced in a simple manner, a more exact proportioning of the inflow of brake fluid to the return-flow pump during drive-slip control is possible. The need to bleed brake fluid to the brake unit via the pressure relief valve too frequently can thereby be avoided, thus protecting both the pressure relief valve and the brake unit. Furthermore, there is no possibility that, in the event of a malfunction of the precharging pump to the effect that this does not cut off, additional brake fluid will be pumped into the brake circuit during the anti-lock control, which could result in damage to the brake unit.
If an overall pressure accumulator chargeable by the precharging pump is provided, then it is possible to use a precharging pump of favorably small construction which, together with the accumulator likewise of small construction, can be integrated into the hydraulic unit of the ASR and ABS, and at the same time the charging mode of the precharging pump can be controlled in a simple way.
If the return-flow pump of the brake circuit of the driven vehicle wheels is configured as a self-priming pump which therefore does not need a precharging pump, a considerable overall simplification of control device construction is achieved.
In turn, a precise proportioning of the outlet pressure of the return-flow pump is possible by an inflow control valve. This valve can be switched jointly with the ASR control valve.
A buffer chamber inserted into the hydraulic periphery of the return-flow pump affords additional safety to the effect that a sufficient quantity of well deaerated brake fluid is available in the drive-slip control mode. In combination therewith, it is especially advantageous to insert the pressure relief valve into the hydraulic unit of the ASR, likewise in order as far as possible to prevent air from being admixed with the brake fluid.
A solenoid valve inserted between the outlet of the return-flow pump and its feed line is actuatable to prevent in a simple manner the possibility that the self-priming return-flow pump will suck brake fluid from the reservoir during an anti-lock control operation.